Hydraulic load limiter



April 9, 1968 M. B. THORNTON 3,376,739

HYDRAULIC LOAD LIMITEE 2 Sheets-Sheet l Filed Oct. l2, 1965 INVENTOR MARSHALL B GIORNTON BY 09], /M/h/ ATTORNEY April 9, 1968 M. B. THOYRNTON 3,376,739

HYDRAULIC LOAD LIMITER Filed Oct. l2, 1965 2 Sheets-Sheet FIG 2 FICL- FIC`15 INVENTOR Wyjm/ United States Patent Otltice 3,376,739 HYDRAULIC LOAD LIMITER Marshall B. Thornton, Granby, Conn., assigner to United Aircraft Corporation, East Hartford, Conn., a corporation of Delaware Filed Oct. 12, 1965, Ser. No. 495,104 Claims. (Cl. 73-141) This invention relates to load limiters and particularly to a load limiter having special advantages for use in a cable of a hoist for lifting heavy articles. The load limiter of this invention was developed for use in the cable of a hoist used in installing or removing an airplane propeller unit from the engine shaft of an airplane and will be described as used for this purpose, although it will Ibe obvious that the load limiter is readily adaptable for other uses.

In handling a propeller unit for airplanes, the hoist cable is provided with a metal ixture for engaging the propeller hubs so that the propeller unit can De raised into position to be installed on the propeller shaft of the airplane engine. Occasionally it happens, when the propeller has -been slipped onto the shaft, a gust of wind acting on the opposite wing of the airplane will result in seriously overloading the hoist mechanism, resulting in damage to the hub engaging iixture or other parts of the hoist mechanism which if not noticed may result in subsequent failure.

It is an object of this invention to provide a load limiter in the cable of a hoist for introducing a limited amount of stroke Within a specified range for eliminating destructive overloads in the event of a largeV extraneous force lbeing applied over a short time period or distance to the object being handled by the hoist.

More specifically it is an object of this invention to provide in series in the hoist cable such a load limiter comprising a hydraulic cylinder communicating with a gas charged accumulator to control pressure exerted by a piston in the cylinder and allow a definite amount of travel of the piston relative to the cylinder upon overload without destructively overloading the hoist equipment.

Another object of ythis invention is to provide in such a load limiter both a visual warning device vfor indicating when an overload has occurred and also for recording that the overload has occurred and the extent of the overload.

A further object of the invention is to provide a periodic audible signal of the motion of the load limiter while the hoist is operating within safe limits of overload and warn of impending destructive overloads before they occur.

Briey the improved load limiter of this invention consists of a single acting hydraulic cylinder and piston sized for the desired load, stroke and pressure. The cylinder, at one side of the piston, is vented to atmosphere while the cylinder at the other side of the piston is -iilled with hydraulic fluid. This iluid is pressurized by one or more accumulators to provide the initial preload, or no-deflection load capacity. The accumulators, pressurized by gas, act as a load limit spring, any movement of the piston forcing fluid into the accumulator, thereby decreasing volume and increasing pressure of the conlined gas. This pressure increase provides the stroke-load ratio, or spring rate, of the system.

A still further object of this invention is generally to improve load limiters of the type described.

These and other objects and advantages of the invention will become apparent or will be pointed out in connection with the following detailed description of a prefcrred embodiment of-the invention shown in the acl companying drawings.

3,376,739 Patented Apr. 9, 1 968 In these drawings:

FIG. l vis a schematic diagram of a cable hoist load limiter of the hydraulic type embodying the invention;

FIG. 2 is an end view of a preferred construction for such a hydraulic load limiter;

FIG. 3 is a longitudinal sectional view of the load limiter of FIG. 2 taken on lines 3-3 of FIG. 2;

FIG. 4 is a sectional view on line 4 4 of FIG. 2;

FIG. 5 is a sectional view on line 5 5 of FIG. 4;

FIG. 6 is a sectional view on line 6-6 of FIG. 4.

As shown schematically in FIG. 1, the load limiter consists of a single acting cylinder 10 and a piston 12. The cylinder has a vent 14 -to atmosphere at one end and has at its other end suitable seals 16 for a piston rod 18 which is extended through this end of the cylinder. The vented end of the cylinder is provided with a cable attaching eye 20 and the external extended end of the piston rod 18 is provided with a similar cable eye 22. As will be evident, the remote end of the cylinder and piston rod have their eyes 20 and 22 connected in the cable of a hoist.

A pipe connection is provided at one end of the cylinder remote from the vent 14 for a hydraulic pipe 24 which communicates with two accumulators 26, 28 through branch pipes 30, 32, respectively. Each accumulator consists of a tank 34 and an expansible bag 36 which is shorter than tank 34 but otherwise conforms to the inside dimensions of the tank, leaving a chamber 38 at one end of the tank with which the branch pipes 30 and 32 are in fluid communication. The -bags 36 have short necks 40 extended through apertures in the tank, housing valves through which the bags are charged with nitrogen gas. Pipe24 is provided with a pressure gage 42 and an oil charging valve 44 by which cylinder 10 and the accumulators are initially filled. Air 'bleed valves 46 are also provided in the branch pipes 30, 32 to bleed air from the system during lling.

Referring to FIGS. 2 and 3 showing a working embodiment of the load limiter, a cylinder 10a has a head 50 welded to it at one end and a cylinder head attaching ring 52 Welded to it at its other end to which a cylinder head 54 is removably attached by cap screws 56. Vents 58 are provided in cylinder head 54 (FIG. 2) and a cable eye 20a is alsoprovided. A piston 12o has a piston rod 18a which extends through vpacking 16a in cylinder head 50 and carries at its extreme end a screw threaded clevis tting 58 to which the propeller attaching iitting is secured by a bolt 62, as indicated (broken away) at 64. An arm 66 carried by the piston rod between shoulder 68 thereon and fitting 58 carries a cord 70 for a bell alarm subsequently to be described. p

lTwo accumulators 26a and 28a are mounted on brackets 72 secured by cap screws 56 at one en-d of the cylinder and on branch pipes 30a and 32a carried by cylinder head 50 and communicating directly with the interior of cylinder 16a. Each accumulator tank contains a bag 36 having a neck 40 through which it is charged with nitrogen gas as in the FIG. 1 showing. A pressure gage` 74 is provided which is mounted directly on cylinder head 50 in communication with cylinder 10a and which indicates the pressure of hydraulic iluid in cylinder 10a and chamber 38 ofthe tanks 26a and 28a.

An audible alarm device 76 is carried by cylinder 10a midway of its length on brackets 78. These brackets carry a rectangular plate`80 which supports a fixed, upright post 82 on the upper end of which a gong 84 is fixed by a nut 85.

At the base of the post a spring well 85a is provided housing a clock-type spring 86 attached at its outer end to a pin 88 on plate 80 and at its inner end to a sleeve 90 rotatable about post 82. Sleeve 90 also provides a' reel 92 (FIG. 5) on which cord 70 is found. The other end of the cord is attached to arm 66 on piston rod 18a (FIG. 3). As piston 12a moves away from cylinder head S4 toward cylinder head 50, cord 70 rotates reel 92 in a clockwise direction (FIG. 4) to Wind up spring 86.

Plate 80 also carries a pedestal 93 at one side, but within the confines of the depending rim of gong 84 on which a striker arm 94 is pivotally mounted on a pin 95. A torsion spring 95a on pin 95 biases a U-shaped actuator having a lower inwardly directed end 95h. The U-shaped actuator carries a depending abutment 95C at its other end in position to engage arm 94. The end 9Sb of this actuator is engaged successively by four cams 96 which project upwardly from the upper plate of reel 92 as the latter is rotated in a clockwise direction (FIG. 4) to move arm 94 against the bias of spring 95a. It will be evident that as the cams 96 move past the end 95h the arm 94 carrying the striker will sound the gong repeatedly to give audible warning of an overload.

While the particular dimensions of the load limiter will vary widely for different applications, the one shown in the drawings for a propeller unit hoist has an effective piston area on its unvented side of 3 square inches and a stroke of l2 inches, giving a total displacement of 36 cubic inches. The accumulator bags are charged at 500 pounds per square inch to insure the proper volume of gas, and the oil in the accumulator tanks is subsequently charged at 600 pounds per square inch, giving a working pressure range of 600-1000 pounds per square inch. T hese figures are given by way of example, as it will be understood that for a different use the -dimensions and pressures chosen may vary considerably.

In the operation of a hoist equipped with the load limiter above described, the 600 pounds per square inch initial preload, or 11o-deflection load, insures that in the normal operation of the hoist the piston of the load limiter remains at the left-hand end of the cylinder (FIG. 3). If for any reason the load on the hoist cable is increased beyond this normal value by any extraneous force of more than 1800 pounds the piston will move in the cylinder, forcing oil into the accumulator chambers and compressing the gas in the accumulator bags. The resulting increase in the pressure in the accumulators provides the stroke-load increase ratio, or spring rate, of the system. With the load limiter illustrated, an increase from 1800 pounds of load to 3000 pounds of load occurs before the piston has reached the end of its stroke and has seated against cylinder head 50.

During this movement of the piston, the pressure gage 74 gives a visual indication of the amount of overload. A r

second hand on the gage may be provided which will record, until manually reset, the amount of overload which has occurred after the load has been reduced to normal and the load limiter has been reset by the pressure in the accumulators.

Also, during the movement of the load limiter piston through what may be termed the tolerable overload range, the alarm bell will be periodically struck as the piston moves in the cylinder to give repeated warnings of the overload condition.

When the piston has reached the end of its travel it will engage the cylinder 50 and prevent further compression of the gas in the accumulator bags. Thus damage will be prevented in the event of severe overloads beyond the designed capacity of the load limiter but within the structural capability of the device. However, long before this condition occurs, ample warning will have been given that an overload exists.

It will be evident that as a result of this invention a load limiter has been provided for a hoist which limits the load on the hoist by providing in the hoist cable sufcient motion upon overload which acts to compress an expansible gas in an accumulator, thus relieving the hoist from destructive overload and providing for resetting the load limiter automatically when the overload is relieved.

it will also be evident that the pressure of the cornpressed gas in the accumulator serves to give a visual indication on a pressure gage so that overload can be instantly detected and its magnitude determined, and that the operation of the load limiter upon overload gives repeated audible warnings that an overload exists.

Further, the initial no-deiiection load of the load limiter as indicated on the gage gives immediate indication of a leak in the system.

It will also be noted that the load limiter of this invention can be used in any attitude, vertical or horizontal, and may be adapted for use with either tension or cornpression loads.

While only one embodiment of the invention has been shown and described, it will be understood that many changes may be made in the construction of the load limiter without exceeding the scope of the invention as dened in the following claims.

I claim:

1. A load limiter for a hoist comprising a cylinder and piston adapted to be connected in series between an object to be lifted and the hoist, means for venting said cylinder to atmosphere on the side of the piston adjacent the hoist, an accumulator communicating with said cylinder on the other side of said piston, means for charging said cylinder on said other side with a hydraulic fluid, and means for charging said accumulator with a compressed gas for pressurizing said hydraulic iluid to provide an initial no-deiiection load on said piston.

2. A load limiter for a hoist comprising a cylinder and piston adapted to be connected in series between the object to be lifted and the hoist, means for venting said cylinder on the side of said piston adjacent said hoist, an accumulator having fluid communication with said cylinder on the other side of said piston including a tank and a bag within said tank, means for charging said cylinder on said other side of said piston with a hydraulic fluid, and means for charging said bag with a gas under pressure to pressurize said hydraulic uid and provide an initial no-deflection load on said piston.

3. In a load limiter for a hoist, a cylinder and piston connected in series between the hoist mechanism and the object to be lifted, said cylinder having means for connecting it at one end to the hoist mechanism and said piston having means at its remote end for connecting it to an object to be lifted, means for venting said cylinder to atmosphere at its connected end, an accumulator including a tank providing a pressure chamber, means providing fluid communication between said chamber and the other end of said cylinder, means for charging said cylinder and said chamber with hydraulic iluid, means for charging said chamber with an expansive gas for providing an initial no-deiiection load on said piston during the normal use of said hoist, and means for providing visual and audible indications of overload as said piston moves in said cylinder.

4. A load limiter for a hoistcomprising a cylinder and piston adapted to be connected in series between an object to be lifted and the hoist, means for venting said cylinder on the side of said piston adjacent the hoist, an accumulator having fluid communication with said cylinder on the other side of said piston including a tank, said tank and said cylinder adapted to contain hydraulic uid, and means for charging said tank with an expansible uid to provide an initial no-deection load on said piston.

5. The load limiter of claim 4 in which an expansible container is provided in the tank for the expansible iiuid, and means is provided for charging the container from outside the tank.

6. The load limiter of claim 4 in which a pressure gage is provided in communication with the hydraulic iiuid for normally indicating the initial no-deection load and for indicating overload pressures as the piston moves in the cylinder.

'7. In a load limiter for a cable hoist, a cylinder and piston adapted to be connected in series in the cable between the hoist mechanism and an object to be lifted, means for venting said cylinder at its end adjacent the hoist, an accumulator mounted on said cylinder including a closed chamber having iluid communication with said cylinder adjacent the other end of said cylinder, means for charging said chamber with a hydraulic Huid, a flexible bag in said chamber, means for charging said bag with an expansible gas to provide an initial no-deection load on the hydraulic fluid acting on said piston, means for giving a visual indication ofthe pressure acting on said hydraulic Huid including a pressure gage, and means for giving an audible warning as the piston moves relative to said cylinder.

8. In a load limiter for a hoist, a cylinder and piston connected between the hoist mechanism and the object to be lifted, said cylinder having means for connecting it at one end to the hoist cable and said piston having means at its remote end for connecting it to the object to be lifted, means for venting said cylinder to atmosphere at its connected end, accumulator means carried by said cylinder including a plurality of tanks arranged about the circumference of said cylinder, said tanks having fluid connections to said cylinder at its other end, and means for providing an initial no-deection load on said piston includingr a .bag in each tank having means external of said tanks for charging `said bags with an expansible tluid under pressure.

`9. The load limiter of claim 8 in which a pressure gage is provided to indicate t-he normal 1ro-deflection load on the piston and also the overload as said piston moves in said cylinder.

10. The load limiter of claim 8 in which a gong is provided having a striker and means for sounding said gong repeatedly as said piston travels in said cylinder under overload conditions.

References Cited UNITED STATES PATENTS 2,641,926 6/1953 Eisenbart 73-141 3,017,939 1/1962 Vegors 73-141 X DAVID SCHONBERG, Primary Examiner. I. W. ROSKOS, Assistant Examiner. 

7. IN A LOAD LIMITER FOR A CABLE HOIST, A CYLINDER AND PISTON ADAPTED TO BE CONNECTED IN SERIES IN THE CABLE BETWEEN THE HOIST MECHANISM AND AN OBJECT TO BE LIFTED, MEANS FOR VENTING SAID CYLINDER AT ITS END ADJACENT THE HOIST, AN ACCUMULATOR MOUNTED ON SAID CYLINDER INCLUDING A CLOSED CHAMBER HAVING FLUID COMMUNICATION WITH SAID CYLINDER ADJACENT THE OTHER END OF SAID CYLINDER, MEANS FOR CHARGING SAID CHAMBER WITH A HYDRAULIC FLUID, A FLEXIBLE BAG IN SAID CHAMBER, MEANS FOR CHARGING SAID BAG WITH AN EXPANSIBLE GAS TO PROVIDE AN INITIAL NO-DEFLECTION LOAD ON THE HYDRAULIC FLUID ACTING ON SAID PISTON, MEANS FOR GIVING AS VISUAL INDICATION OF THE PRESSURE ACTING ON SAID HYDRAULIC FLUID INCLUDING A PRESSURE GAGE, AND MEANS FOR GIVING AN AUDIBLE WARNING AS THE PISTON MOVES RELATIVE TO SAID CYLINDER. 